The Quebrada Blanca Phase 2 copper mining Project owned by Teck, SMM/SC and ENAMI includes the construction of a 160km-long 2x220kV electrical transmission line to bring power to the mine located in the Atacama Desert in Chile’s Tarapacá Province. Transelec owns the line which has an alignment that crosses the Quebrada Guatacondo dry river bed, 170km south-east of Iquique, the closest city to the project site.

In this context, Bessac, with sister company Soletanche Bachy Chile, oversaw the design and construction of a 2.5m-diameter, 898m-long utility microtunnel bored in a single drive.

CHOOSING MICROTUNNELLING

Initially, Transelec proposed the use of an earth pressure balanced (EPB) tunnel boring machine with segmental lining and an overall length of 840m. But this faced several constraints and difficulties:

  • The TBM would require a long launching shaft of around 50m in length to satisfy the logistical requirements of service trains and muck-removal cars.
  • Due to environmental permits, the layout of the launching area was very limited and the arrangement of the authorised surface layout would not allow logistics management during excavation of the tunnel along the proposed axis. This would mean modifying the launch alignment, increasing the overall length to 875m, and adding a curve in the horizontal plane.
  • With an EPB TBM, the logistics between the machine and the launch shaft are typically met by a service train equipped with muck cars. Because these trains can only accommodate a limited gradient, up to around 3% -3.5%, it would require deepening the launch shaft from 10m to 18m, and the exit shaft from 28m to 40m.

For the above reasons, the Joint Venture proposed an alternative solution which kept the required 2.5m internal tunnel diameter. A Herrenknecht slurry microtunnel boring machine (MTBM) would bore the tunnel which would be lined with jacked pipes. This solution has several advantages:

  • The launch shaft internal diameter was maintained at 11m.
  • The depth of the launch shaft was maintained at 10m
  • The area required on the surface for equipment was reduced.
  • The MTBM can operate on gradients up to 8.5%, so there was no need to deepen the shafts.
  • All mobilisation and launching stages of the MTBM are quicker than a TBM solution and so reduce the overall project schedule. Given these benefits, the microtunnelling approach was the most time- and cost-effective solution.

To dip under the riverbed, the tunnel has a banana-shaped profile. This comprises an initial downward slope of 8%, followed by an upward slope of 8.5%. After the first downward slope of 8%, the longitudinal profile was designed with a gradually lower slope in order to make the low-point smoother. This proved to be beneficial as it facilitated machine guidance and also limited the opening of the jacking pipes. During the execution stage, the exit shaft position was shifted and the tunnel length increased to 898m.

At 2.5m internal diameter (ID) and 3m OD, this is a large microtunnel, and the substantial drive length all resulted in a challenging project with additional constraints described below.

A REMOTE DESERT LOCATION

The closest city to the project is Iquique, located 170km away – around a three-hour drive. So one of the main difficulties for managing the project was global supply and logistics. And there was no water supply. To ensure a constant supply of water in the volumes required, large tanks were set up on site; rented 15m3 tankers brought daily supplies from Iquique. The scope of work also included cleaning and watering of nearby roads.

In order to recycle as much water on site as possible and avoid the disposal of mud slurry, it was decided to treat the slurry after microtunnelling and reuse the water for cleaning the roads. A system of centrifuge and flocculation was used with additional storage tanks. With this system in place, the recycled water complied with Chilean specifications for watering water in terms of pH, turbidity and other chemical parameters. Thanks to this innovative environmental process, the site saved many tanker journeys to and from Iquique, and it also had the benefit of using less water:

  • 100% of water recycled from the microtunnelling was reused for road watering, so no water tankers were required specifically for this purpose.
  • No trucks were used to remove slurry mud from site.
  • To ensure the required amount of fuel, large-capacity fuel tanks were set up on site and supplied by rented tankers from Iquique. One of the main risks on this kind of remote project is that of potential delays due to the lack of spare parts, such as cutting tools and other items. A large and redundant consignment stock of spare parts, cutting tools, consumables and various products was kept on site to meet any potential risks.
  • The project was undertaken at the same time as the Coronavirus pandemic, with general mobilisation starting in March 2020 and a microtunnelling breakthrough occurring in January 2021. All the difficulties relating to the remoteness of the project were exacerbated by the Covid measures, with visa restrictions, quarantine for foreigners and transport issues. It was therefore even more important to anticipate all logistics and spare-part stocks to avoid any site stoppages.

GEOLOGICAL CONDITIONS

During the tender stage, compact alluvial sandy gravels with a granulometry of up to 300mm (12in) were anticipated but the geotechnical investigations and existing boreholes carried out were not as deep as the tunnel depth. The gravel content varied between 20% and 80%, the sand content varied between 50% and 80%, and the fines content between 5% and 20%. This heterogeneous granulometry required the ability to quickly adapt the slurry fluid parameters according to the geology encountered and to adapt the slurry flow according to the density of the excavated material.

Indeed, there is a risk that slurry density increases due to fines content but slurry disposal in this area and the supply of fresh water to create new slurry fluid was very difficult. The separation system was designed for maximum recycling of slurry, with a flocculation unit, a centrifuge and various storage tanks. Before the start of excavation, laboratory tests were undertaken to accurately adapt the slurry fluid component. The formula had also been tested on site with water used for excavation to validate the composition.

GRANULOMETIC CURVE FOR QUEBRADA BLANCA AREA

The proposed MTBM was equipped with a mixed cutterhead. During the excavation of the shafts, larger boulders than anticipated – up to 600mm in diameter – were encountered. To avoid the risk of cutterhead blockages, crushing difficulties and loss of front stability, it was decided to adapt the cutterhead geometry on site. Sixteen additional bars were added to the cutterhead to limit the size of opening to 200mm in order to keep the largest elements in front of the cutterhead. These bars were designed by the Bessac design office and manufactured in Chile. Laboratory tests were undertaken on the boulders extracted from the shaft excavation in order to define their mechanical characteristics.

On some sections, ground cover was low, especially at the launching area where it was only 5m – less than twice the diameter of the MTBM. To limit the risk of settlement or slurry blow-out, the slurry density was slightly increased in such sensitive sections of the alignment; MTBM excavation parameters were also adjusted in these areas (front slurry pressure, contact force, slurry flow, etc).

CHILEAN SAFETY CONTEXT

The project was required to follow Chilean mining safety standards covering several key occupations, such as crane operator, rigger and excavator operator. Without any one of these key operators, the site would grind to a halt. Moreover, the local standards involve a system of ‘one person – one task’ which makes project management more difficult.

With the Covid crisis, the risk of suspected or positive cases was very real. The management decided to hire spare people for each key position to mitigate the risk of having the project come to standstill, as the pipe-jacking process should never be interrupted for a long period. To achieve 24/7 working, production was based on four teams, two shifts of 12 hours with a rotation every 14 days, and with base camp located 3km from the site. Thanks to its previous experience in a similar context a few years ago, Bessac could anticipate these high safety requirements to execute the project properly and on time.

EXCAVATION OF THE TUNNEL

The MTBM was launched from an 11m internal diameter shaft and recovered from one that was 8m ID. Both shafts were excavated with conventional methods, using a 12t excavator and a mobile crane equipped with a 2m3 grabber. The lining of the shaft comprises a 250-300mm thickness of reinforced shotcrete.

The main jacking frame in the shaft had a thrust capacity of 1,400t, with eight intermediate jacking stations implemented along the tunnel, each of which could develop a thrust force of 1,150t. Of the eight, only three were used during the jacking process, especially at the end of the drive.

All the equipment was chosen to ensure that adequate thrust capacity would enable the construction of a pipejacked tunnel with a relatively long alignment and a large external diameter for this technique.

INTERMEDIATE JACKING STATION

For a long-drive tunnel and especially one where the profile has a ‘banana’ shape, lubrication is very important. Three lubrication injection lines were installed in the tunnel, one for the MTBM and two for the tunnel. The advantage of using two separated lines for the tunnel is that each lubrication operator is responsible for around a 400m length of tunnel for closer monitoring of the process. In addition, there is real-time monitoring with flowmeters installed on the pump on the surface and displayed on the control container screen at the surface.

At the beginning of the excavation, not all Bessac key people could have been on site due to Covid-19 visa restrictions, transport restrictions and lockdown. Some of the local operators received distance learning from Bessac experts.

At the start of excavation, without all the expatriate specialists of Bessac, the efficiency of the process was reduced. The average excavation rate rose when all Bessac operators were present on site, with daily bests of up to 22m/day. Thanks to the site’s stock of spare parts, all breakdown and maintenance issues could be resolved in situ to enable a quick restart of excavation.

As the pipes had a 3m external diameter, their unit length was 2.5m to maintain standard sizes for transportation. Usually, jacking pipes have two lifting anchors located on their upper portion. But for this project, the pipes had four lifting anchors, two on each side. This was beneficial as it made anchors accessible from ground level without workers having to clamber at height to reach them, making pipe handling safer and saving time.

SIDE-MOUNTED PIPE LIFTING ANCHORS

To manage drainage water in the tunnel, a dewatering pump was installed at the lowest point and moved as the excavation progressed. Once the drive was completed, removing all the lines inside the tunnel was more complex due to the steep gradients. To facilitate the task and improve safety, a hydraulic winch was installed in the launch shaft and a special Bessac-designed trolley was used to accelerate this task. This trolley is specifically designed to transport all equipment, including the slurry pipe, electrical cables, the hydraulic cylinders that are part of the first jacking station and the brackets.

CONCLUSION

With breakthrough occurring on 7 January 2021, the completion of the Atacama microtunnel marks Bessac’s fifth project in Chile. But the challenges faced, particularly the technical constraints associated with the relatively large dimensions of the microtunnel and its location in a desert environment, involved complex logistics. Undertaking the project during a full-blown public health emergency only served to exacerbate these challenges. Thanks to its expertise, its capacity to adapt and its worldwide experience, Bessac was able to complete this project successfully.